Dislocation Interface Interactions Research Articles

Understanding Dislocation-Interface Interactions during Recrystallization of Mg-(Ca, Zn) Alloys

Understanding Dislocation-Interface Interactions during Recrystallization of Mg-(Ca, Zn) Alloys

The dependence of strength and strain hardening on dislocation-interface interaction in dual-phase titanium alloys

The slip transmission across an interface is essential for the mechanical properties of dual-phase alloys like Ti-6Al-4 V. However, the correlation between the dislocation-interface interaction and the strength and strain hardening anisotropy remains unclear due to the lack of direct experimental evidence. Via in situ scanning electron microscopy micropillar compression, prismatic plane dislocations were preferentially activated and interacted with an individual α/β interface at different angles. Based on transmission electron microscopy characterization, this study suggests that α/β interface shows a more pronounced strengthening effect when the coordinated slip system is more difficult to be activated and the slip deflection angle is larger. Differently, its higher strain hardening rate is initially determined by the larger Burger vector magnitude of interfacial residual dislocation after slip transmission. These results provide a unique basis for understanding the contribution of the interface to the mechanical properties of dual-phase alloys.

Role of interfaces on the mechanical response of accumulative roll bonded nanometallic laminates investigated via dislocation dynamics simulations

Unraveling the effects of continuous dislocation interactions with interfaces, particularly at the nanometer length scales, is key to a broader understanding of plasticity, to material design and to material certification. To this end, this work proposes a novel discrete dislocation dynamics-based model for dislocation interface interactions tracking the fate of residual dislocation on interfaces. This new approach is used to predict the impact of dislocation/interface reactions on the overall mechanical behavior of accumulative roll bonded nanometallic laminates. The framework considers the dynamic evolution of the interface concurrent with a large network of dislocations, thus, accounting for the local short and long range effects of the dislocations under the external boundary conditions. Specifically, this study focuses on two-phase Fe/Cu nanometallic laminates, and investigates the role of the underlying elastic and plastic contrast of the Fe and the Cu layers on the composite response of the material. Moreover, the role of initial microstructures, resulting from processing is also investigated. Subsequently, the model is used to examine the effect of layer thickness and interface orientation relationship on the residual stresses of the relaxed microstructure. The associated mechanical response of these laminates are compared when loaded under normal direction compression, as well as shear compression. Finally, this work predicts a dominant effect of the layer thickness, as compared to the interface orientation relationship, on the macroscopic response and on the residual stresses of these nanolaminates, while the local dislocation transmission propensity through the interface is significantly influenced by the corresponding orientation relationship.

Unique nanostructures and deformation mechanisms in a high-entropy alloy nanolaminates

Introducing nanolaminates structure is a promising strategy to obtain the controllable microstructure and mechanical properties of high entropy alloy (HEA). In this work, the Al0.5CoCrFeNi/CoCrCuFeNi nanolaminates with coherent interface and crystalline/amorphous hierarchical nanostructure was designed. On the one hand, the coherent interface with continuous slip direction and planes enables nanolaminates with great deformability. On the other hand, the hierarchical nanostructure in the nanolaminates contributes to high strength via the interaction between crystalline and amorphous structures. Microstructural observations indicated that abundant nanotwins and stack faults were filled in the columnar nanocrystalline cutting across the interface, leading to a high hardness due to the dislocation-interface interactions. The results of nanoindentation tests revealed that the hardness of the nanolaminates up to 8.02 GPa, which markedly exceeds many HEA films with face-centered cubic (FCC) phase. The findings provide a new avenue for improving the mechanical properties of HEA nanolaminates in a controllable manner.

Atomistic modeling of interface strengthening in Al-Si eutectic alloys

Al-Si cast alloys are usually composed of α-Al and Al-Si eutectic. Si flakes and Al matrix generally hold cube-on-cube orientation relationship with the primary interface (111)Al∥(111)Si. Extensive experimental studies demonstrated that Si flakes cannot significantly improve mechanical properties of Al-Si cast alloys. We hypothesize that the weak strengthening effect associated with Si flakes might be attributed to thermomechanical properties of Al-Si interfaces besides their morphologies. To characterize Al-Si interfaces with a large lattice mismatch (> 30%), we proposed the quasi-coincident site lattice (Q-CSL) as reference lattice, and demonstrated that the Q-CSL Al-Si coherent interface has three characteristic coherent structures, one stable and low energy structure and two metastable and high energy structures. The translation vectors for the same type of coherent Q-CSL structures are consistent with three displacement shift complete (DSC) vectors. The two metastable structures can be obtained by shifting the low energy structure with three partial DSC vectors. Semi-coherent interface is composed of the low energy Q-CSL patches and three sets of interface misfit dislocations with Burgers vectors same as the DSC vectors. Atomistic simulations revealed that Al-Si interface exhibits low shear resistance. Ideal shear strength of the Q-CSL coherent interface is 110 MPa and semi-coherent interface is 20 MPa. The low shear resistance is attributed to the glide of interface misfit dislocations. Al-Si interface also exhibits low formation and migration energies of point defects. Owing to low shear strength and low formation and migration energies of point defects, interface sliding or shear readily happen under mechanical loading or during dislocation-interface interactions. Lattice dislocations can cross slip onto or climb along Al-Si interfaces. These reactions decrease the number of accumulated dislocation loops around Si flakes and promote nucleation and emission of lattice dislocations from Al-Si interfaces to matrix, consequently reduce the repulsive force on approaching dislocations and weaken Si flakes strengthening effect. In situ tension and compression tests in a scanning electron microscope reveal relatively weak strengthening effect due to Si flakes, consistent with the computed dislocation interaction with interfaces and shear behavior of interfaces.

Screw dislocation impingement and slip transfer at fcc-bcc semicoherent interfaces

Understanding of dislocation-interface interactions at the atomistic level could provide useful insights into the mechanisms behind slip transfer, a key information for higher length-scale formulations. A systematic study of screw dislocation impingement on both Fe/Al and Fe/Cu interfaces with Kurdjumov-Sachs orientation relationship has been carried out using atomistic simulations. For the “strong” Fe/Al interface, there is appreciable dislocation delocalization, in which the change in dislocation content provided by Nye tensor analysis. Interestingly, the delocalization is significant enough that large simulation cells must be employed in order to avoid boundary effects. When the impinging dislocations are far away from the boundaries, the large accumulation of shear strains nearby the impinging dislocations at the interface causes dislocation nucleation/emission at these places. While for the “weak” Fe/Cu interface, the dislocation delocalization at the interface is notably long-range, extending more than 20 nm away from where the initial dislocations entered the interface.

Martensitic transformation induced dislocation walls in Fe42Mn38Co10Cr10 high-entropy alloy

Two types of hitherto unreported dislocation walls in a cold-deformed Fe42Mn38Co10Cr10 (at.%) high-entropy alloy are characterized using aberration-corrected scanning transmission electron microscopy at the atomic-scale. Both walls are located at the interface between two parallel and juxtaposed martensite lamellae, although they are respectively constituted by all-30° mixed or all-90° edge Shockley partial dislocations. Within each of these two walls, two different sets of Shockley partials that have Burgers vectors of the same magnitude but opposite signs are arranged alternately on successive basal planes of the hexagonal lattice. The interfaces containing such compact dislocation walls are expected to contribute to strain hardening and plastic accommodation via various dislocation-interface interactions.

Nanoindentation characterization of strengthening mechanism in a high strength ferrite/martensite steel

A 0.1C5Mn3Al steel with laminated ferrite/martensite and layered ferrite/martensite microstructures was processed by hot-rolling, cold forging and subsequent annealing. Tensile tests revealed that hot rolled sample (laminated) and cold forged + 650 °C annealed sample (layered) showed similar tensile strengths but different elongations. It is confirmed that laminated structure is good for retaining elongation. Furthermore, nanoindentation tests were performed at locations at or near interfaces and in the ferrite or martensite matrix, revealing that the nanohardness difference across the interface is important in affecting the elongation of the material. The underlying mechanisms were discussed with regarding to the dislocation-interface interactions.

Glass formation by severe plastic deformation of crystalline Cu|Zr nano-layers

We use molecular dynamics simulation to study the formation of glasses during severe plastic deformation of crystalline Cu|Zr nano-layers in a simple shear geometry. The early stage of plasticity is governed by dislocation plasticity. Dislocation-interface interaction induces interface roughening and slow atomic-level intermixing around interfaces. A solid-state crystal-to-amorphous transition starts near the interfaces. At high strain, the interfaces are refined to merge, leading to the formation of an intermixed glassy zone that eventually spans our simulation cell. Dislocation emission is then suppressed and strain is fully accommodated by this glassy phase. We characterize the crystal-to-amorphous transition using local structural measures, such as the pair distribution function and Voronoi analysis. The onset of glass formation occurs at a critical shear strain that increases with layer thickness. Our simulations indicate that the parameter controlling this onset is the theoretical affine interface distance, i.e. the amorphization occurs at a critical value of around 3–4 nm that increases slightly with the increase of layer thickness. Our work provides insights on mechanically driven glass formation in multicomponent systems, as e.g. realized in ball-mills or high-pressure torsion experiments.

Nanoindentation and nanoscratching of a ferrite/austenite iron bi-crystal: An atomistic study

Abstract Molecular dynamics simulations are applied to investigate the wear/friction behavior of a ferrite/austenite iron bi-crystal, as a model system for duplex stainless steels. The plasticity of the ferrite phase is dominated by dislocations while both dislocations and stacking faults are the primary cause of plastic deformation of the austenite phase. Interestingly, the responses of tribological parameters vary depending on the scratch direction. For instance, the scratch hardness is increased by about 46% whereas the friction coefficient is reduced by about 22% when scratch starts from austenite to ferrite. At the interface, a local softening/hardening occurs because of dislocation-interface interaction. The present results demonstrate that martensitic phase transformation is responsible for experimentally observed high amount of ferrite of the pile-up.

Computing forces on interface elements exerted by dislocations in an elastically anisotropic crystalline material

Driven by the growing interest in numerical simulations of dislocation–interface interactions in general crystalline materials with elastic anisotropy, we develop algorithms for the integration of interface tractions needed to couple dislocation dynamics with a finite element or boundary element solver. The dislocation stress fields in elastically anisotropic media are made analytically accessible through the spherical harmonics expansion of the derivative of Green’s function, and analytical expressions for the forces on interface elements are derived by analytically integrating the spherical harmonics series recursively. Compared with numerical integration by Gaussian quadrature, the newly developed analytical algorithm for interface traction integration is highly beneficial in terms of both computation precision and speed.

Concurrent interface shearing and dislocation core change on the glide dislocation-interface interactions: a phase field approach

Strengthening in nanoscale metallic multilayers is closely related to the glide dislocation-interface interaction. The interface can be sheared by the stress of the approaching glide dislocation with its core changed. How the concurrent interface shearing and the dislocation core change influence such interaction dominated strength is studied using three dimensional phase field microelasticity modeling and simulation. The simulated results show that when the glide dislocation is close to or away from the interface, the width of its core changes abruptly in accompany with the interface shear zone broadening or shrinking, respectively. A wider interface shear zone is developed on the interface with a lower shear strength, and can trap the glide dislocation at the interface in a lower energy state, and thus leads a stronger barrier to dislocation transmission. The results further show that the continuum model of the dislocation without the core-width change underestimates the interfacial barrier strength especially for the glide dislocation transmission across weak interfaces.

Plastic instability mechanisms in bimetallic nanolayered composites

Strain localization is a common deformation-induced instability in many metallic metals. How it happens is related to both microstructure and the way in which plasticity is mediated prior to localization. Both aspects can fundamentally change in a face-centered cubic metal when it becomes nanostructured; the propensity for deformation twinning increases and the behavior is dominated by dislocation–interface interactions. Here we carry out a transmission electron microscopy investigation to elucidate the collaborative role of deformation twinning and dislocation transmission on the onset of strain localization in nanolayered composites. Two material systems are examined, Cu–Ag and Cu–Nb, and for each system, two interface structures are examined, one prone to dislocation transmission and the other not. We show that dislocation transmission favors crystallographic band formation, whereas dislocations that do not transmit cause interface tilting and are associated with (non-crystallographic) shear band formation.

Atomistic modeling of defect-induced plasticity in CuNb nanocomposites

CuNb nanocomposites have proven stable under light-ion irradiation, although their response to heavy-ion bombardment and subsequent loading is still uncertain. In this paper we analyze the change in mechanical properties of model CuNb nanocomposites in vacancy supersaturated environments, mimicking certain effects introduced by heavy-ion irradiation. We have performed compression tests using molecular dynamics for different defect contents. The presence of defects substantially modifies the system response upon external loading. The dislocation nucleation mechanism changes with the interface atomic density. Stacking fault tetrahedra in the Cu layer may act as dislocation sources to lower significantly the yield stress and the same applies for voids present in the Nb layer. We have analyzed in detail the dislocation-interface interaction mechanisms under different conditions, showing how dislocations react with the misfit dislocations present at the interface to modify the atomic structure of the boundary, which suggests that the interface could be designed to optimize its defect healing properties. We conclude that heavy-ion irradiation softens the multilayer nanocomposites and that interfaces help in recovering these materials as they absorb the dislocations created from irradiation-induced defects.

The effect of interfaces on the mechanical behaviour of multilayered metallic laminates

The mechanical response of multilayered metallic laminates is dominated by size effects through the confinement of dislocation motion within the layers. We deconvolute the contributions to the plastic behaviour resulting from dislocation–dislocation interactions and dislocation–interface interactions, using discrete dislocation dynamics and atomistic simulations. Upper and lower bounds for the material strength are found by considering two limiting cases for the influence of the interfaces: hard and shearable. Hard interfaces, preserving interfacial dislocations, are shown to significantly increase the strength of the multilayered metallic laminates, whereas a deformable interface results in lesser hardening. Molecular dynamics simulations show that the {1 1 1}Cu ∥ {1 1 0}Nb Cu/Nb interface response lies between these two cases. Additionally, the plastic response of Cu/Nb multilayered metallic laminates is studied and shown to be isotropic due to an effect of averaging among layers, despite the plastic anisotropy of the respective layer materials.

Structure and shear deformation of metallic crystalline–amorphous interfaces

The structure and shear properties of crystalline–amorphous laminar nanocomposites are studied in an atomistic model of face-centered cubic copper with amorphous Cu46Zr54 bulk metallic glass in the quasi-static limit. The plastic shear deformation response is determined by the production and motion of interface dislocations at the crystalline–amorphous interface, which is closely linked to the structural and chemical transition from crystalline Cu to the amorphous Cu/Zr phase. The implication of interfacial shear are discussed in context of dislocation–interface interactions and co-deformation of a crystalline–amorphous nanocomposite.

The heterophase interface character distribution of physical vapor-deposited and accumulative roll-bonded Cu–Nb multilayer composites

We present a method for characterizing the full five parameter heterophase interface character distributions (HICD) using two-dimensional electron back-scatter diffraction (EBSD) images. We apply the HICD method to determine the orientation relationships and three-dimensional normal vectors of Cu–Nb interfaces in both physical vapor-deposited (PVD) pure Cu–Nb (4μm individual layer thickness) and accumulative roll-bonded (ARB) alloyed Cu–Nb multilayer composites (200–600nm layer thickness). The HICD analysis shows that {112}Cu planes are most preferentially and frequently bonded with {112}Nb planes with Kurdjumov–Sachs and Nishiyama–Wasserman misorientations in the ARB alloyed Cu–Nb multilayers. These interfaces differ from the {111}Cu||{110}Nb interfaces predominantly found in the PVD pure Cu–Nb multilayered thin films. Also, pure tilt type interfaces with a [111]/30° misorientation and {110}Cu planes bonded to {112}Nb planes were found in ARB alloyed Cu–Nb multilayers. In the ARB material the observed Cu–Nb interfaces differ from what would be obtained from random pairings of the Cu and Nb orientations in terms of the relative intensities (in multiples of random distribution) and shapes of the interface normal peaks, which indicates that these interfaces were preferentially selected during the high strain ARB process. The measured ARB textures along the interface also differ from the theoretical rolling textures for each bulk single phase metal, suggesting that during ARB layer refinement these interfaces have some influence on slip activity by constraining grain deformation or through the kinetics of dislocation–interface interactions.

Analysis of plastic deformation in nanoscale metallic multilayers with coherent and incoherent interfaces

Nanoscale metallic multilayered (NMM) composites possess ultra high strength (order of GPa) and high ductility, and exhibit high fatigue resistance. Their mechanical behavior is governed mainly by interface properties (coherent and/or incoherent interfaces), dislocation mechanisms in small volume, and dislocation–interface interaction. In this work, we investigate these effects within a dislocation dynamics (DD) framework and analyze the mechanical behavior of two systems: (1) a bi-material system (CuNi) with coherent interface and (2) a newly developed tri-material system (CuNiNb) composed of both coherent and incoherent interfaces. For the bi-material case we analyze the influence of networks of interfacial dislocations whose nature and distribution are commensurate with the level of relaxation and loading of the structure. Misfit and pre-deposited interfacial dislocation arrays, as well as combinations of both, are studied and the dependence of strength on layer thickness is reported, along with observed dislocation mechanisms. It is shown that interfacial defect configurations significantly alter the strength and mechanical behavior of the material. Furthermore, it is shown that the implementation of penetrable interfaces in DD captures the strength dependence at layer thicknesses on the order of 3–7 nm. For the tri-material case we analyze the effects of coherent and incoherent interfaces in large-scale simulations. The results show that these materials have strong strength-size dependence but are limited by the strength of the incoherent (CuNb) interface which is weak in shear. The weak interface acts as a dislocation sink. This in turn induces an internal shear stress field that activates cross-slip in the adjacent CuNi interlace and thus causing softening. Moreover, it is shown that the yield stress of the CuNiNb system is controlled by the volume fraction of the Nb. Because Nb is the most compliant of the three materials, an increase in volume fraction of Nb decreases the overall yield strength of the material.

The influence of interface shear strength on the glide dislocation–interface interactions

Interfaces with relatively low shear strengths can be strong barriers to glide dislocations due to dislocation core spreading within the interface plane. Using atomistic modeling we have studied the influence of interface shear strength on the interaction of lattice glide dislocations with fcc/bcc interfaces. “Tunable” interatomic potentials are employed to vary the interface shear strength for the same interface crystallography. The results show that: (1) the interface shear strength increases as the dilute heat of mixing decreases; (2) the interface shear mechanism involves the nucleation and glide of interfacial dislocations, which is dominated by the atomic structures of interfaces, regardless of the interface shear strength; (3) weak interfaces entrap lattice glide dislocations due to the interface shear and core spreading of dislocations within interfaces. Reverse shear displacement is needed to enable collapse of the spread core for slip transmission. This study provides an insight into the correlation between interface shear strength and glide dislocation trapping at the interface, which is a crucial unit mechanism in understanding the ultra-high strengths observed in nanoscale fcc/bcc multilayers.

In situ TEM observations of room temperature dislocation climb at interfaces in nanolayered Al/Nb composites

Using in situ nanoindentation , we investigated dislocation–interface interactions in Al/Nb multilayers. Preferential storage of dislocations at interfaces, as opposed to within layers, was observed. Recovery of dislocations was observed to occur through climb in the interfaces. The rapid climb of dislocations is ascribed to high vacancy diffusivity and vacancy concentration in the interfaces. The vacancy formation energy at interfaces, 0.12 eV, as estimated from the experimentally measured climb rates, was found to be significantly lower than in the bulk.